¾cupmelted and cooled semisweet or bittersweet chocolate
In a medium heavy saucepan, heat 1 cup of the sugar and the water over medium heat, stirring until the sugar is dissolved. Stop stirring. Using a pastry brush dipped in water to wash down any sugar crystals that may have formed on the side of the pan.
While sugar is dissolving, beat egg whites on medium speed in an electric mixer using the whip attachment until foamy. Add the cream of tartar and beat until the whites form soft peaks. Gradually beat in the remaining ¼ cup sugar until stiff peaks form.
Increase the heat for the sugar syrup to high and boil the mixture until roiling bubbles form and a candy thermometer reads 240 degrees Fahrenheit.
With the mixer on medium-high speed, slowly pour the hot sugar syrup into the egg whites, being careful not to pour directly onto the whip attachment.
Continue beating on medium speed until the bottom of the mixing bowl feels barely warm to the touch.
On medium speed, slowly add the softened butter, about a tablespoon at a time, and continue beating at medium high speed until fully incorporated. The buttercream may first appear to be curdled, then will become light, smooth, and fluffy.
On low speed, add the cooled melted chocolate, and beat until well-incorporated.
Remove the bowl from the mixer, and using a rubber spatula, continue blending in any unmixed chocolate that may appear. Use frosting immediately.
Fills and frosts 2 10-inch cake layers, generously
Chocolate Italian Buttercream - http://chefgailsokol.com/cakes-and-frostings/chocolate-italian-buttercream/